Reversible covalent modification

Reversible covalent modification is the making and breaking of a covalent bond among a nonprotein group and an enzyme molecule. While a range of nonprotein  groups  may  be  reversibly  attached  to  enzymes  that  affect  their activity, the mainly general modification  is the removal and addition of a phosphate group phosphorylation and dephosphorylation, respectively. The Phosphorylation   is catalyzed   through protein   kinases,   frequently using ATP as the

 Figure: Plot of initial velocity (V0 ) against substrate concentration for the allosteric enzyme aspartate transcarbamoylase.

                              Figure:   The reversible phosphorylation and dephosphorylation of an enzyme.

phosphate donor and dephosphorylation is catalyzed through protein phosphatases in above figure. The removal and addition of the phosphate group causes modification in the tertiary structure of the enzyme which alter its catalytic activity.  One class of the protein kinases transfers the phosphate  specifically  on to the hydroxyl group of Thr or Ser residues on the goal enzyme serine or threonine protein kinases, typified   through   3′,5′-cyclic    adenosine    monophosphate    (cAMP)-dependent   protein kinase, although a second class transfers  the phosphate  on to the hydroxyl  group of Tyr residues tyrosine kinases. The Protein phosphatases catalyze the hydrolysis of phosphate groups from proteins to regenerate not modify hydroxyl group of the amino acid and release Pi which is shown in figure.

A phosphorylated enzyme may be either less or more active than its dephosphorylated form.Therefore phosphorylation or dephosphorylation may be used as a rapid reversible that is switched to turn a metabolic pathway on or off according to the requirement of the cell. For instance glycogen phosphorylase an enzyme included in glycogen breakdown is active in its phosphorylated form and   glycogen synthase, included in glycogen synthesis is much active in its unphosphorylated form.

Other kinds of reversible  covalent  modification  which are used to regulate  the activity  of  certain  enzymes  involves  adenylylation the  transfer  of  adenylate from  ATP  and  ADP-ribosylation the  transfer  of  an  ADP (adenosine diphosphate)-ribosyl  moiety from NAD.